Foil bearings are currently used in high-speed turbomachinery, and particularly in small air cycle machines where air is used as the lubricant. This eliminates the need for an oil lubrication system, lowering the cost and eliminating maintenance and reliability problems associated with oil systems. Compared to the small air cycle machines, gas turbine engines produce higher loads on the bearings, requiring larger bearings which generate more heat due to fluid friction. Yet the temperature limit for `state of the art` foil bearings remains the same. The limit is set by the organic material used for coating the foils, which has the purpose of low rubbing friction and tolerance to foreign particles. When the temperature limit is exceeded, experience has shown that the coating material deteriorates and results in rubbing which ultimately produces bearing failure.
Foil bearings in small air cycle machines are cooled by conduction to relatively cool parts of the machine and by forcing cool air to flow through the bearings. In gas turbine engines, parts adjacent to the bearings are not always cooler than the bearings need to be. Also, the cooling air pressure required to force enough air through the bearings may be higher than the available pressure in the engine, especially during operation at high altitudes. Even in cases where enough pressure is available, this method of cooling is not very efficient because most of the air flow bypasses the bearing through the foil retaining slots. In gas turbine engines, cooling air must be used efficiently because of the associated performance penalty, which can increase engine weight and cost.
Heat generation in foil bearings is concentrated in the regions of smallest film thickness, i.e. where the air film separating the foil from the rotating component is less than 0.001 inch thick. Heat must be transferred away from these regions; radially in the case of journal bearings and axially in the case of thrust bearings. The foils are separated from the bearing housing by springs and air gaps to allow for dynamic motion and thermal expansion of the rotating part. This causes high resistance to heat transfer into the bearing housing. The rotating component, however, is always in intimate thermal contact with the air film where heat is generated by viscous shear, and the contact is distributed uniformly over the circumference due to its rotation. Therefore, to facilitate the heat transfer along the path of least resistance, i.e. into the rotating part and achieve the highest possible cooling effectiveness, the rotating part must be cooled directly.
Accordingly, a need exists for an apparatus for providing cooling air flow to the rotating components of journal and thrust foil bearings in gas turbine engines.